1. Field of the Invention
The present invention relates to apparatus and method for performing measurements of the earth from within a borehole.
2. Description of the Related Art
Exploration for energy such as hydrocarbons or geothermal sources generally requires drilling a borehole into the earth. The borehole can be used to gain access to depths of the earth for performing measurements related to the exploration for energy.
Well logging is a technique used to perform the measurements from the borehole. In well logging, a logging tool is conveyed through the borehole. The logging tool includes those components used to perform the measurements. In one embodiment, a wireline is used to support the logging tool and to transmit the measurements to the surface of the earth for processing and recording.
Many types of measurements can be performed from within the borehole. Some of these measurements use an enclosed instrument cell as a component of the logging tool. The enclosed instrument cell generally has a diaphragm that provides an interface with the environment external to the cell. For example, an enclosed instrument cell can be configured to measure the pressure exerted on the diaphragm. The pressure exerted on the diaphragm is transferred to components internal to the cell, which measure the pressure.
When the logging tool is disposed deep in the borehole, the tool can be exposed to high pressure and high temperature. For example, pressure in the borehole can exceed 10,000 psi and the temperature can exceed 300° C. In general, the enclosed instrument cell is filled with a liquid to equalize the pressure internal to the cell to the external pressure to prevent the cell from being crushed at the high pressure. However, the high temperature can adversely affect the liquid causing the instrument cell to malfunction.
Therefore, what are needed are techniques to measure properties of the earth from within a borehole at high pressures and high temperatures.